A case where a PNC leaves a piconet without performing a normal PNC handover procedure is called “PNC failure.”
A device with PNC capability serves as a new PNC when a PNC failure occurs. This operation is called “PNC appropriation.” Piconet devices with P-CTA allocated participate in the PNC appropriation process, which are called “participating devices.”
There is a continuing demand for provisioning for delay-stringent data traffics, such as audio/video (A/V) signals, in a wireless network environment. Examples of wireless communication protocols, which makes it possible to provide Quality of Service (QoS) required by applications susceptible to a transmission delay, are “IEEE 802.15.3 Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs)” and “IEEE 802.15.4 Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs).”
A piconet refers to a wireless network of devices that share a common coordinator and operate in accordance with the IEEE 802.15.3 or similar standards, which includes one PNC and a plurality of devices around the PNC. The PNC performs an important operation of managing the piconet, and data transmission timing is given by a superframe.
The example in FIG. 1 illustrates the structure of a piconet superframe specified in the IEEE 802.15.3 standard.
A superframe of the IEEE 802.15.3 is composed of three parts: a Beacon 100, a Contention Access Period (CAP) 102, and a Channel Time Allocation Period (CTAP) 104.
The beacon 100 is used to provide data transmission timing in a piconet, to set the timing allocations, and to transmit management information to a plurality of piconet devices. The CAP 102 is used to communicate command packets and asynchronous data between the PNC and piconet devices. The CTAP 104 is composed of a plurality of Channel Time Allocations (CTAs) 106 and 108 that are used to transmit commands, isochronous data, and asynchronous data. The CTAs are classified into ordinary CTAs and Pseudo-static CTAs (P-CTAs).
Data traffics in a piconet can be classified into the following types:                Asynchronous data type: This type usually does not have constraints on packet delivery time, an example of which is E-mail.        Synchronous data type: This type is sensitive to a packet transmission delay but is not strict in requirements for a packet transmission delay.        Interactive data type: A packet transmission delay of which can be sensed by a user but does not adversely affect the usability or functionality.        Isochronous data type: This type is susceptible to a packet transmission delay and an excessive packet transmission delay adversely affects usability and functionality. A typical example is data that is generated when A/V signals are transmitted.        
FIG. 2 illustrates an example of a piconet that is specified in the IEEE 802.15.3 standard.
The piconet includes: a PNC 200 for managing the piconet; and a plurality of piconet devices 202, 204, 206 and 208. The piconet devices are classified into devices with PNC capability and devices without PNC capability. That is, all of the piconet devices do not need to have all functionalities defined in the IEED 802.15.3 standard. For example, in a scenario where a computer and a portable storage device are wirelessly connected using the IEEE 802.15.3 technology, the computer can be implemented as a device with PNC capability and the portable storage device can be implemented as a simple device without PNC capability.
The PNC 200, which is responsible for starting, managing, and ending a piconet, broadcasts beacon frames at the beginning of every super-frame to the devices in the piconet so as to provide them with timing and piconet information. The information transmitted by the beacon frame includes the channel time allocation information, that is, which CTA is allocated to which pair of devices.
In general, the PNC 200 may change the location of a CTA within the superframe every superframe. Accordingly, a device failing to receive a beacon cannot transmit data. In order to prevent a case where the devices in the piconet fails to receive a beacon and thus a data throughput decreases, the device can request allocation of a special type of CTA called pseudo-static CTA (P-CTA) to the PNC. A device with a P-CTA allocated to itself can transmit data for a predetermined time duration using the P-CTA even after it failed to receive a beacon. The reason for this is that the location of the P-CTA within a superframe is guaranteed to remain static for a pre-determined number of superframes. For example, in the case of the IEEE 802.15.3 standard, the location of the P-CTA does not change at least for a time duration corresponding to “mMaxLostBeacon” superframes.
When a PNC wants to leave the piconet for some reason, it follows a PNC handover procedure so that the piconet can keep operating without interruption to its services. However, it is not always possible for a PNC to follow a PNC handover procedure before leaving the piconet due to various unpredictable situations such as abrupt power failure/off, hardware or software failure, sudden departure from the radio communication range of the piconet, and so on. The event of PNC leaving a piconet without following a proper handover procedure is called “PNC failure” as described above.
In the event of PNC failure, the piconet is left without a PNC and all the data traffics are interrupted due to the lack of coordination. To resume their communication, the remaining devices must follow a series of steps such as radio channel scanning, starting a new piconet, association, and CTA request/allocation. However, even when the devices can successfully resume their communications, following these steps will leave the piconet services seriously impaired. Especially, this kind of interruption may be unacceptable for isochronous data type, a typical data type for A/V signals.
When a piconet is in operation, the devices in the piconet are either idle or involved in one or more data transmissions either as a source or a destination device. The devices in the idle state, that is the devices with no packet to transmit, are affected little by the PNC failure. The devices with no packet to transmit have only to construct a new piconet when a need for packet transmission arises.
In the case of the devices transmitting non-isochronous data, data transmission is somewhat affected by the PNC failure. However, this problem can be solved by the upper layers of the Open System Interconnect (OSI) network stack in most cases.
On the other hand, in the case of the devices transmitting isochronous data, the PNC failure and the resulting interruption of data communication can lead to a service failure. What is therefore required is a scheme for minimizing the impact of the PNC failure on the transmission of the isochronous data.
An example of a conventional scheme for solving this problem is “A Seamless Coordinator Switching (SCS) Scheme for Wireless Personal Area Networks” proposed by Won-Soo Kim, Il-Whan Kim, Seung-Eun Hong and Chung-Gu Kang and published in “IEEE Transactions on Consumer Electronics (Vol. 49, No. 3, 554-560 pp, 2003. 8).”
The SCS scheme makes it possible to minimize the overhead due to piconet re-initialization caused by the PNC failure. In the SCS scheme, a PNC continuously manages and updates a list of PNC candidate devices along with the order of precedence of the devices, and broadcasts this information to devices in the piconet. The PNC determines the order of precedence of PNC candidate devices using information such as “PNC capability” defined in the IEED 802.15.3 standard. Each of the PNC candidate devices memorizes its order of precedence S. Each PNC candidate device decreases the memorized order of precedence S by 1 when it fails to receive a beacon for a predetermined time duration. If S reaches zero, the device takes on the responsibility of PNC.
The SCS scheme is advantageous in that it can be implemented by adding a few functions to the IEEE 802.15.3 standard. However, in case a hidden terminal problem exists, multiple devices may declare themselves as the next PNC, or a device may try to take on the PNC responsibility when the current PNC is operational.
The SCS scheme can reduce an overhead due to re-initialization when the PNC failure really occurs. However, when the “Hidden Terminal” problem exists, a healthy piconet may collapse. Moreover, it may take excessively a long time until the next PNC comes forward when the PNC failure occurs, which may seriously interrupt or terminate a service for isochronous data.
An example of a conventional method for solving these problems of SCS is “Active Seamless Coordinator Switching (ASCS) Scheme for Fast PNC Handover in WPAN” proposed by He-Jin Nam, Chong-Ho Yoon and Young-Ae Cheon and published in “Proceedings of the 8th conference on Next Generation Communication Software”, pp. 216-219, 2004. 12.
Differently from SCS scheme depending only on received data, the ASCS scheme can minimize the impact of the PNC failure by transmitting a probe message in the CAP from a device failing to receive a beacon. However, the ASCS scheme conflicts with the IEED 802.15.3 standard that prohibits message transmission in the CAP when a device fails to receive a beacon.